Consumers of nitrogen in the electronics industry typically require ultra-high purity nitrogen which contains less than 1 part-per-billion (ppb) of any contaminant such as oxygen, hydrogen and carbon monoxide. Concentrations of these substances in nitrogen obtained from a conventional cryogenic air separation plant are typically in the range of about 0.5-2 parts-per-million (ppm). Oxygen, which has a higher boiling point than nitrogen, is almost completely removed by the cryogenic distillation. However, since the boiling point of carbon monoxide is very close to that of nitrogen, and that of hydrogen is much lower, most of the carbon monoxide present in the feed air to the cryogenic air separation plant is also present in the nitrogen product stream from the plant, and the hydrogen concentration in the nitrogen product stream is about double that in the feed air.
Removal of these contaminants is typically carried out using a conventional adsorption process following the cryogenic air separation process. However, such a system is disadvantageous because of the large size of the adsorption vessels needed to carry out the purification.
An alternative to the use of conventional ambient temperature adsorption processes for producing ultra-high purity nitrogen is the upstream oxidation of hydrogen and carbon monoxide to water and carbon dioxide, respectively. These oxidation products are then removed in a molecular sieve prepurification system prior to the cryogenic air separation. This oxidation is typically carried out as a catalytic process. A major disadvantage of this oxidation process is that it requires high temperatures, increasing the energy requirements, and hence the cost of the entire process. Another disadvantage is that the oxygen remaining in the product nitrogen stream must be removed by another means, usually a separate cryogenic distillation process, which adds further to the cost of the overall process.
Accordingly, it is an object of this invention to provide an improved system for producing ultra-high purity nitrogen.